1. Field of the Invention
The present invention relates to a carbon coated plastic electrode for use in batteries. More particularly, the present invention relates to a coating which may be applied to a carbon plastic electrode and which provides a high electrochemical surface area.
2. Description of the Prior Art
Electrodes are key components in batteries. As will be understood, certain types of batteries, employ carbon plastic electrodes. Accordingly, it has long been known that an improved carbon plastic electrode would result in improved battery performance.
In this regard, batteries of the prior art typically include a stack of cells, and an electrolyte, or liquid containing positive and negative ions, that facilitates the flow of electricity therethrough. Each battery cell includes an electrode upon which an anodic reaction occurs, and an electrode upon which a cathodic reaction occurs. As a result of these separate reactions, energy is either stored or released. In batteries which are known as monopolar batteries, each electrode functions as a single pole, so that an anode and a cathode are required to form an individual cell. Further, an ion-permeable barrier, or separator, separates the anode from the cathode. In these monopolar batteries, the cells forming the stack are hydraulically isolated from each other and electrically coupled in series by an external electrical conductor. As should be understood, electrons leave the cathode, travel ionically through the electrolyte and separator, and are deposited on the anode. The electrons then leave the body of the battery, travelling electronically through the electrical conduit to the cathode of the adjacent cell.
In bipolar flow batteries, each electrode includes two poles, that is, an anodic reaction occurs on one side of the electrode and a cathodic reaction occurs on the opposite side of the same electrode. Each cell of a bipolar flow battery is electronically coupled in series and hydraulically coupled in parallel to an adjacent cell. In this arrangement, electrons travel ionically through the electrolyte and through an ion-permeable separator. Further, the electrons flow electronically between adjacent cells and through the bipolar electrode, which is common to both the anodic and cathodic half cells. Thus, in this type of battery, electron flow is entirely internal. In contrast, electricity leaves the interior of a monopolar battery as it travels through the external electrical conduit. As should be understood, the electrical conduit is a relatively high electrical resistance path. As a result, bipolar batteries generally possess a higher current density than monopolar batteries because electrons in bipolar batteries do not have to travel along the relatively high resistance, external path.
Accordingly, bipolar batteries and, in particular, bipolar, zinc-bromine batteries, have proven to be superior to other types of battery design for applications where high current and energy density are required.
Zinc-bromine batteries have an aqueous solution of zinc-bromide and quaternary ammonium salts, for example, methylethylpyrrolidinium bromide, with optional supporting salts, such as NH.sub.4 Cl, which is circulated through the individual cells from an external reservoir. Each cell has two portions, one half of the cell contains an anolyte and the other half of the cell contains a catholyte. The anolyte flows through a common anolyte manifold to each anodic half cell and the catholyte flows through a parallel common catholyte manifold. The alternating separators and electrodes are sealed together in a manner which prevents communication between the anolyte and catholyte systems.
In the discharged state, the anolyte is substantially chemically identical to the catholyte. During the process of collecting a charge, the following chemical reaction takes place: EQU Zn.sup.++ +2e.sup.- .fwdarw.Zn EQU 2Br.sup.- .fwdarw.Br.sub.2 +2e.sup.-
Zinc is plated on the anode, and bromine is produced at the cathode. The bromine is immediately complexed by the quaternary ammonium ions in the electrolyte to form a dense second phase which is subsequently removed from the battery stack with the flowing electrolyte. Further, when the battery is charged, zinc is stored on one side of each electrode and the complex bromine is stored in the catholyte reservoir.
During the electrical discharge process, the following chemical reaction takes place. EQU Br.sub.2 +2e.sup.- .fwdarw.2Br.sup.- EQU Zn.fwdarw.Zn.sup.++ +2.
In this reaction, zinc is oxidized, and the released electrons pass through the bipolar electrode where they combine with molecular bromine to form bromide ions. The positively charged zinc ions travel through the separator and remain in solution, and at the same time, bromide ions pass through the separator in the opposite direction and remain in solution.
As will be recognized following a study of the description above, the electrodes are key elements in batteries because it is upon their respective surfaces that critical electrochemical reactions take place. Electrodes for use in a bipolar battery may be formed from extruded, carbon-filled, high-density polyethylene or carbon plastic. Sheets of carbon plastic may be affixed by molding or other similar techniques in plastic frames. Such a frame may then be thermally welded to other components to form a battery as is described more fully in U.S. Pat. No. 4,945,019 and U.S. Pat. No. 5,308,718, the disclosures of which are incorporated by reference herein.
Of some importance in a bipolar battery design which employs a plastic-component construction is the rate at which the bromine-bromide reaction takes place. When compared to the zinc-plating reaction, the bromine-bromide reaction on a carbon plastic electrode is relatively slow.
It has been found that the rate of the bromine-bromide reaction may be increased by coating the surface area of the electrode on which the reaction takes place. In particular, it has been found that certain coatings have improved the efficiency and cycle life for zinc-bromine batteries.
For example, high surface area carbon coatings applied to carbon plastic electrodes have been associated with improved efficiencies and were expected to yield a very high electrochemical surface area and, therefore, increase the rate of the bromine-bromide reaction. However, this has not always been the case.
Accordingly, it would be desirable to have an improved electrode for use in a bipolar battery wherein a bromine-bromide reaction could occur at a relatively faster rate. It would also be desirable to have a coating, which when applied to an electrode, produces a high electrochemical surface area, when compared to the prior-art techniques. It would also be desirable to have an improved electrode which improves the efficiency and cycle life of a bipolar battery which incorporates same.